1. Field of the Invention
The present invention relates to a solid-state imaging device, a method of manufacturing the solid-state imaging apparatus, and an imaging apparatus. More particularly, the invention relates to a solid-state imaging device having a photoelectric conversion element, a method of manufacturing the solid-state imaging device, and an imaging apparatus equipped with the solid-state imaging device.
2. Description of Related Art
Developments of cameras for image input that are used with personal computers have been carried out increasingly in recent years. Solid-state imaging devices that are incorporated in such cameras employ CCD image sensors, which use charge coupled devices (CCD), and CMOS image sensors, whose manufacturing process is compatible with CMOS manufacturing processes.
A CCD image sensor is an image sensor in which photoelectric conversion elements (photodiodes) corresponding to pixels are arrayed two-dimensionally and respective pixel signals turned into electric charge by the photoelectric conversion elements are read sequentially using vertical transfer CCDs and horizontal transfer CCDs. A CMOS image sensor is similar to the CCD image sensor in the point that photoelectric conversion elements corresponding to pixels are arrayed two-dimensionally. However, the CMOS image sensor does not use vertical and horizontal transfer CCDs for signal reading, but it reads respective signals stored in respective pixels from selected pixels by select lines made of aluminum or copper wiring lines, like a memory device. Although the CCD image sensor and the CMOS image sensor are different in reading systems for pixel signals and so forth, their photodiodes, serving as photoelectric conversion elements, have a common structure.
An example of known structure of the photodiode is shown in FIG. 16 (see JP-A-2002-170945). FIG. 16 shows a state in which a photodiode 103 is formed between element-isolating regions 102 formed on a surface layer portion of a silicon substrate 101. The photodiode 103 has a structure in which impurity regions, namely, a P+ region 104, an N+ region 105, an N− region 106, and a P− region 107, are formed in that order from the surface of the silicon substrate 101 in a depth direction. The symbols “+” and “−” in the figure indicate that a case where the impurity concentration is “higher” than other regions and a case in which the impurity concentration is “lower” than other regions, respectively.
Such a structure makes it possible to reduce dark current produced from the surface of the silicon substrate 101 of the photodiode 103. Electron-hole pairs are generated by the light entering the region of the photodiode 103, and signal charges (electrons) are stored in the junction portion of the P region and the N region. The maximum value of the signal charge that can be stored in the PN junction portion is referred to as a saturated signal charge amount (hereinafter also denoted as “Qs”). An image sensor with high Qs is excellent in various characteristics such as dynamic range and SN (signal/noise) ratio. Accordingly, increasing of Qs is a very important factor in achieving improvements in the characteristics of the image sensor.